Python 3.4+Async programmingterm

asyncio.Lock

synchronization

An asynchronous mutex: while one task holds the lock, the others wait at async with without blocking the event loop.

Syntax

lock = asyncio.Lock()
async with lock: ...

You need a lock only where an await sits between checking and updating shared state: without an await tasks do not switch and there is no race. threading.Lock in a coroutine stalls the whole loop and asyncio.Lock does not, but it is not thread-safe. It is not reentrant: a nested async with lock in the same task waits for itself forever. Acquire it only with async with — a plain with lock raises TypeError.

Examples

import asyncio
counter = 0
async def add(lock):
    global counter
    async with lock:
        value = counter
        await asyncio.sleep(0.01)  # переключение здесь — без замка вышла бы гонка
        counter = value + 1
async def main():
    lock = asyncio.Lock()
    await asyncio.gather(*(add(lock) for _ in range(5)))
    print(counter)  # → 5
asyncio.run(main())

# Python 3.11+
import asyncio
async def main():
    lock = asyncio.Lock()
    try:
        with lock:
            pass
    except TypeError:
        print('нужен async with')  # → нужен async with
    await asyncio.sleep(0.01)
asyncio.run(main())

See also

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